Electrolysers use electricity to transform reactant chemicals to desired product chemicals through electrochemical reactions, i.e., reactions that occur at electrodes that are in contact with an electrolyte. Hydrogen is a product chemical of increasing demand for use in chemical processes, and also potentially for use in hydrogen vehicles powered by hydrogen fuel cell engines or hydrogen internal combustion engines (or hybrid hydrogen vehicles, also partially powered by batteries). Electrolysers that can produce hydrogen include: water electrolysers, which produce hydrogen and oxygen from water and electricity; ammonia electrolysers, which produce hydrogen and nitrogen from ammonia and electricity; and, chlor-alkali electrolysers, which produce hydrogen, chlorine and caustic solution from brine and electricity.
Water electrolysers are the most common type of electrolyser used to produce gaseous hydrogen. The most common type of commercial water electrolyser currently is the alkaline water electrolyser. Alkaline water electrolysers utilize an alkaline electrolyte (typically an aqueous solution of, e.g., 25% to 35% KOH) in contact with appropriately catalyzed electrodes. Hydrogen is produced at the surfaces of the cathodes (negative electrodes), and oxygen is produced at the surfaces of the anodes (positive electrodes) upon passage of current between the electrodes. The rates of production of hydrogen and oxygen are proportional to the current flow in the absence of parasitic reactions and stray currents and for a given physical size of electrolyser. The electrolyte solute (potassium hydroxide) is not consumed in the reaction, but its concentration in the electrolyte may vary over a range with time, as a result of discontinuous replenishment of water reacted and also lost as water vapour with the product gases.
As used herein, the terms “half cell”, “half electrolysis cell” and equivalent variations thereof refer to a structure comprising one electrode and its corresponding half cell chamber that provides space for gas-liquid (electrolyte) flow out of the half cell. The term “cathode half cell” refers to a half cell containing a cathode, and the term “anode half cell” refers to a half cell containing an anode.
As used herein, the terms “cell”, “electrolysis cell” and equivalent variations thereof refer to a structure comprising a cathode half cell and an anode half cell. A cell also includes a separator membrane (referred to herein after as a “membrane”), typically located between, and in close proximity to or in contact with, the cathodes and anodes. The functionality of the membrane is to maintain the hydrogen and oxygen gases produced separate and of high purity, while allowing for ionic conduction of electricity between the anode and cathode. A membrane therefore defines one side of each half cell. The other side of each half cell is defined by an electronically conducting solid plate, typically comprised of metal, and generally known as a bipolar plate. The functionality of the bipolar plate is to maintain the fluids in adjacent half cell chambers of adjacent cells separate, while conducting current electronically between adjacent cells. Each half cell chamber also contains an electronically conducting component generally known as a current collector or current carrier, to conduct current across the half cell chamber, between the electrode and the bipolar plate.
Practical (commercial) alkaline water electrolysers utilize a structure comprising multiple cells, generally referred to as a “cell stack”, in which the cells typically are electrically connected in series (although designs using cells connected in parallel and/or series also are known). A cell stack typically consists of multiple cells, with bipolar plates physically separating but electrically connecting adjacent cells. As used herein, the term “structural plate” refers to a body which defines at least one half cell chamber opening and at least two degassing chamber openings. A cell stack typically is constructed using a series of structural plates to define degassing chambers, and alternately cathode and anode half cell chambers for fluid (gas-liquid mixtures and liquid) flow. The structural plates also hold functional components, which may include, for example, cathodes, anodes, separator membranes, current collectors, and bipolar plates, in their appropriate spatial positions and arrangement. The series of structural plates and functional components typically constitutes a filter press type structure, including end (and in some cases, intermediate) pressure plates. The gases generated at the electrodes form gas-liquid mixtures with electrolyte in the half cell chambers, which typically are collected at the exits of the half cell chambers. The gas-liquid mixtures must be treated in degassing chambers, which serve to separate the respective gases from the entrained electrolyte. The terms “electrolyser module” or “electrolyser” refer to a structure comprised of an electrolyser cell stack and its associated degassing chambers.
Most practical water electrolyser modules today utilize large steel vessels located above the cell stack as degassing chambers (also commonly known as gas-liquid separators). There are two general design approaches for circulating fluids in an electrolyser module (i.e., for circulating gas-liquid mixtures from the cell stack to the degassing chambers, and then returning degassed liquid from the degassing chambers to the cell stack).
In the first general design approach, gas-liquid mixtures from each cathode half cell are collected in a manifold above the half cell chambers in the top part of the cell stack, which is connected to the corresponding (hydrogen) degassing chamber via a pipe or tube external to the cell stack; a similar arrangement is used for the anode half cells and the corresponding (oxygen) degassing chamber. The separated liquid is returned from the degassing chambers via piping or tubing that is external to the cell stack to a manifold or manifolds located in the cell stack, beneath the half cell chambers, from which liquid electrolyte is fed back into the individual cathode half cell chambers. There are two main corresponding practical (commercial) sub-approaches.
In the first sub-approach, exemplified in U.S. Pat. No. 4,758,322, the separated liquid in the degassing chambers is mechanically pumped back into the cell stack. While mechanical pumping overcomes the pressure drops in the horizontal manifolds in the cell stack and the external piping or tubing, and allows for large numbers of cells in a single stack (e.g., 200 or more cells), there are several associated disadvantages. For example, the use of a pump adds complexity, capital and operating cost, maintenance requirements, and may adversely affect the availability of the electrolyser module. The pump generally is operated at all times during module operation at a liquid flow rate corresponding to that required for the maximum nominal gas production rate, resulting in maximum associated power losses. Although a dual mechanical pump electrolyser module configuration also is disclosed, typically in practical (commercial) electrolyser modules, a single mechanical pump circuit is used to circulate liquid collected from both degassing chambers back to both the cathode half cell chambers and anode half cell chambers; this maintains equal pressures on either side of the membrane in each cell, but typically adversely affects gas purities by introducing the other gas (entrained in the returning liquid) into both the anode and cathode half cell chambers.
In the second sub-approach, exemplified in U.S. Pat. No. 6,554,978, the anode and cathode fluids are kept separate by relying on gas lift [buoyancy] and gravity head to circulate the fluids in separate circuits without pumps. Advantages of this design approach are the potential to maintain high gas purities and inherently self-regulating fluid flows; however, the number of cells per cell stack is limited by the pressure drop across the horizontal manifolds in the cell stack and the external piping or tubing, and the available vertical space to provide pressure head. Note that the sizes of the manifolds and the conduits connecting the manifolds to the individual half cells are limited by the requirement to restrict stray currents. Consequently, this particular approach generally has been limited to relatively small production capacities, with an associated requirement to use multiple cell stacks or multiple complete electrolyser modules to reach higher production capacities.
In the second general design approach, gas-liquid mixtures from each half cell chamber are fed to the corresponding degassing chamber via gas-liquid feed conduits for each individual half cell chamber. The separated liquid is returned from the degassing chamber via external piping or tubing to a manifold located beneath the half cell chambers, which feeds liquid electrolyte back into the individual half cell chambers. This approach, while somewhat more scalable in terms of the number of cells in a single cell stack, requires a significant amount of piping and assembly, with many mechanical connection points, each representing a potential leak point. Furthermore, scalability remains limited by pressure drops across the common degassed liquid return path, i.e., the external piping or tubing and manifold beneath the half cell chambers in the bottom portion of the cell stack. Electrolyser modules using the second general design approach typically utilize mechanical pumps to circulate the fluids.
In all of the above approaches, the physical size of the electrolyser module, i.e., its lack of compactness for any given hydrogen gas production capacity, is problematic. In an attempt to obtain a more compact electrolyser module, developmental designs that incorporate the degassing chambers into the same structure as the cell stack also have been disclosed. However, none of these designs addresses the other drawbacks described above.
For example, WO 2006/060912 describes a design that incorporates the degassing chambers into the same structure as the cell stack, which also has manifolds above the half cell chambers to collect gas-liquid mixtures from the individual half cell chambers, and bottom manifolds to distribute degassed liquid from the degassing chamber back to the individual half cell chambers. U.S. Pat. No. 2,075,688 and US 20070215492 also describe designs that incorporate the degassing chambers into the same structure as the cell stack, and also teach the use of manifolds beneath the half cell chambers to distribute degassed liquid to the individual half cell chambers. While the anode and cathode half cells are maintained completely separate in these designs, the number of cells per stack is limited by the pressure drop across the horizontal manifolds, and the limited head available in the relatively compact module design.
In order to address the shortcomings of known practical electrolyser modules, what is needed is an inherently scalable design approach, that provides freedom to vary the number of cells over a wide range to meet a wide range of gas production capacity, including very high gas production capacity, while at the same time minimizing associated mechanical connections and assembly, eliminating requirements for mechanical pumping of electrolyte, and maximizing product gas purities. Such a design, especially when further designed to provide a wide range of gas production capacity per cell, would be especially useful when connected to a source of electricity with variable output power, for example, a wind farm or a solar array.